It is now well known to reduce or eliminate shock hazard by providing electrical tools and applicances with three-wire power cords terminating in three-prong plugs adapted to be plugged into three-wire grounded receptacles.
Such three-prong plugs have two power prongs, corresponding generally in structure and function to the prongs of the plugs used in connection with conventional two-wire electrical systems, and a third or grounding plug adapted to engage the grounded female contacts of three-wire grounded receptacles.
Unfortunately, many electrical receptacles commonly found in homes and other buildings are of the two-wire ungrounded variety, rather than the three-wire grounded variety. That is to say, many electrical receptacles commonly found in use today are not provided with a grounded contact and corresponding opening adapted to interfit with the grounding prong of a three-prong plug. Adapters are often used to make it possible to utilize the three-prong plugs with conventional two-wire receptacles, but many times such an adapter cannot be found when needed.
This problem is commonly resolved by cutting off the grounding prong of a three-prong plug, thus making it possible to plug the three-prong plug into a two-wire receptacle, but at the same time "accepting" the attendant shock hazard of an ungrounded system. After being thus mutilated, the three-prong plug can still be plugged into a three-wire receptacle, but cannot be used to take advantage of the shock protection offered by the grounded contact in the third opening.
One known method of dealing with this problem is to make the grounding prong removable from the plug. However, the removed grounding prong is often lost. Another known method of dealing with this problem is to mount the grounding prong of a three-prong plug so it can be retracted, either axially or pivotally, to an inoperative position for use with a two-prong receptacle, and then moved back to operative position for use with a three-prong grounded receptacle. The pivoting method has not been entirely satisfactory because of the poor electrical conducting properties of the simple type of pivot generally used and which is subject to wear and corrosion, sometimes resulting in a faulty ground connection and failure to protect from electrical shock when the user believes he is protected.
In certain countries, such as the United States, convertible electrical plugs of the type under discussion must meet certain requirements imposed by regulating bodies such as the Underwriters Laboratories (UL).
To obtain UL approval, the grounding prong must be self-restoring, that is, it must automatically move back to its extended, active position whenever the device is removed from a two-prong receptacle. Previous inventors have made the grounding prongs axially retractable and have provided them with springs tending to urge the prongs to extended, active position, see U.S. Pat. No. 3,924,914. However, in this construction if the spring is made strong enough to force the grounding prong into the grounding aperture in a three-prong receptacle, the same spring tends to push against the receptacle and extract the power prongs from a two-prong receptacle. Attempts have been made to overcome this problem by providing a removable pin engageable through a hole in the grounding prong and bearing against the plug housing to hold the prong in its active, extended position, as exemplified in U.S. Pat. No. 3,786,392. The separate pin is particularly subject to loss.
U.S. Pat. No. 3,685,000 provides a peripheral groove in the grounding prong engageable by a hook on a grounding wire extending from the plug. However, these grounding wires are notoriously subject to wear and after a few uses can easily break and be detached from the plug. In each of these instances, the absence of the means for holding the grounding prong in extended position tends to result in the aforesaid problem of the spring pulling the power prongs out of the receptacle.
An attempt has been made to provide an internal latch in the grounding prong which is released when the end of the prong is urged against the receptacle plate, see U.S. Pat. No. 3,754,202. However, the mounting of the latch internally of the grounding prong necessarily results in a tiny and complicated mechanism.
These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention.